In the preparation of fields for planting of agricultural crops, the aim of soil preparation is to bury a crop residue and weeds from a previous crop, loosen the soil, and provide a suitable tilth for the crop to be planted. Many crops require relatively small soil particles on the uppermost layer of the soil, at a depth of a few centimeters to tens of centimeters from the surface, this being the depth at which the seed for the crop is sown.
Conventional methods of soil preparation include a primary cultivation and a secondary cultivation. In the primary cultivation, crop residue from the previous crop, and weeds are buried within the soil, and in the secondary cultivation, an uppermost layer of the soil is broken down into relatively small sized particles.
The most popular tool for carrying out the primary cultivation for hundreds of years has been the conventional plough. The plough inverts the soil, giving excellent burial of surface residues. However, there are disadvantages with the conventional plough. Firstly, because the soil is in contact with the plough share, mould board and skimmer, there are high levels of friction in drawing the plough through the soil. Where the plough is towed by a wheeled tractor, because the force necessary to pull the plough is transmitted to the ground through the tractor wheels, there may occur wheel slippage in wet conditions, which causes the soil underneath the tractor wheels to smear and compact, in the bottom of the furrow. The smeared and compacted soil impedes drainage from the furrow.
Secondly because a plough body turns soils over into a space left by a preceding plough body, it is not possible to start at one side of a field and work up and down a field, without the use of a reversible plough which has a set of left hand mould boards for ploughing down a field in one direction, and another opposite set of mould boards for ploughing up the field in an opposite direction.
The reversible plough is expensive, and has the operational complication of switching between mould boards for ploughing in different directions.
A third problem with the conventional plough is that as tractors increase in size, farmers require ploughs with more furrows. As each furrow must be staggered on the plough frame, the total length of the plough frame becomes unmanageable.
Once the primary cultivation of the land has been completed by ploughing, the secondary cultivation is carried out on the soil structure let by the plough. Machines for secondary cultivation include trailed or powered harrows, which break clods by striking, cutting or crushing, to reduce clod size and consolidate the seed bed.
FR-A-2566615 describes a machine for burying stones under finer soil to reclaim stony land. The machine uses a vibrating sieve to grade the stones. However, the machine does not cultivate the soil in so far as the soil is not broken up into smaller particles.
U.S. Pat. No. 2,785,613, U.S. Pat. No. 2,110,997 and U.S. Pat. No. 1,786,226 describe machines for the preparation of farmland. Each discloses means for breaking the soil such as by means of a hammer mill, a shaker and a toothed cylinder respectively. However, in each case objectionable material, such as stones, is conveyed to a dump box or the like.